He guarded 4 positions in college, so it isn't totally unreasonable to think that he can guard 3 in the NBA. There will be some 3's he can guard, some he can't. I don't see him guarding a guy like LeBron or Melo though. LeBron would post him up, Melo would school him from midrange.
That said, he can have a big impact without necessarily guarding the best opposing player. He can play the passing lanes and provide help D. And plus, there's way more teams now who lean on their PG's to create offense than there are that rely on their wing players.